Relaxing the cosmological moduli problem by low-scale inflation
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Springer
Received: January Revised: March Accepted: April Published: April
13, 15, 17, 24,
2019 2019 2019 2019
Shu-Yu Ho,a Fuminobu Takahashia,b and Wen Yinc a
Department of Physics, Tohoku University, Sendai, Miyagi 980-8578, Japan b Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), UTIAS, WPI, The University of Tokyo, Kashiwa, Chiba 277-8568, Japan c Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea
E-mail: [email protected], [email protected], [email protected] Abstract: We show that the cosmological abundance of string axions is much smaller than naive estimates if the Hubble scale of inflation, Hinf , is sufficiently low (but can still be much higher than the axion masses) and if the inflation lasts sufficiently long. The reason is that the initial misalignment angles of the string axions follow the BunchDavies distribution peaked at the potential minima. As a result, the cosmological moduli problem induced by the string axions can be significantly relaxed by low-scale inflation, and astrophysical and cosmological bounds are satisfied over a wide range of the mass without any fine-tuning of the initial misalignment angles. Specifically, the axion with its decay constant fφ = 1016 GeV satisfies the bounds over 10−18 eV . mφ . 10 TeV for Hinf . 10 keV–106 GeV. We also discuss cases with multiple axions and the QCD axion. Keywords: Cosmology of Theories beyond the SM, Beyond Standard Model, Compactification and String Models, Supersymmetric Standard Model ArXiv ePrint: 1901.01240
c The Authors. Open Access, Article funded by SCOAP3 .
https://doi.org/10.1007/JHEP04(2019)149
JHEP04(2019)149
Relaxing the cosmological moduli problem by low-scale inflation
Contents 1 Introduction
1
2 Cosmological moduli problem 2.1 Cosmological abundance 2.2 Astrophysical and cosmological constraints
3 3 6 9 9 10 12
4 Discussion and conclusions
14
1
Introduction
Light axions may be ubiquitous in nature. In string theory, there often appear (relatively) light scalar moduli through compactification [1]. If supersymmetry (SUSY) survives below the compactification scale, a modulus forms a chiral supermultiplet, X. Its lowest component can be decomposed as X = r + iφ, where r and φ denote the real and imaginary parts, respectively. We identify φ with an axion which enjoys discrete shift symmetry, φ → φ + 2πfφ ,
(1.1)
where fφ is the decay constant of the axion. While some of the moduli may drive inflation in the early Universe, explain the current accelerated cosmic expansion or become (a part of) dark matter (DM), others can cause serious cosmological problems [2, 3]. The cosmological impact depends on the moduli masses fixed by the moduli stabilization mechanism. While most of the moduli fields are known to be stabilized by fluxes [4, 5], some of them remain light, and they are stabilized by nonperturbative and/or SUSY breaking effects [6]. For instance, in the KKLT mechanism [7], the K¨ahler modulus is stabilized by instantons/gau
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